It . It is very important that you keep your doctor aprised of your condition. Causes of blood in the urine can be attributed to infection in the kidney or bladder or trauma to these organs. As you fell down the stairs, it makes sense to attribute blood in the urine to trauma.
Also, the stress that your body underwent during and after the fall could make you more succesptible to infection.
See your doctor about the pain on your side and lower back. Let your doctor know about the pain in your urine. It is important you get assessed to ensure no further damage to your body or to follow up on residual tissue damage.